1. Field of the Invention
The subject of the present invention is a valve and, in particular, a valve that can be controlled to deliver a pulsed flow of gas at its outlet.
The expression xe2x80x9cpulsed flowxe2x80x9d is to be understood as meaning that this flow alternates between a high level and a low level during predetermined periods of time resulting from the application of a control signal, generally in the form of square waves
2. Description of the Related Art
Valves which can be controlled to make them supply a pulsed flow at their outlet may find numerous applications, particularly in installations for the pulsed supply to burners of the oxyfuel type. An installation such as this is described in particular in document EP 524 880.
As mentioned in that document, it has in fact been demonstrated that if a burner were to be supplied with a pulsed flow, at least as regards either its fuel or its oxygen supply, it would be possible to obtain a very significant reduction in the nitrogen oxide content of the residual flue gases from the burner. A valve may be fitted to the fuel, particularly natural gas, supply or to the pipe supplying the oxygen supply, typically oxygen, or to both pipes, depending on the installation. As is also described in the aforementioned document, the pulsation frequency is preferably below 1 Hz. Furthermore, in order to obtain a significant effect of reducing the oxides of nitrogen produced, it is necessary for the flow rate or pressure of pulsed gas to have a shape as close as possible to the square waves corresponding to the signals used to control the valve or valves used.
Such valves can also be used for supplying burners with air by way of a source of oxygen.
Depicted in the appended FIGS. 1a and 1b is one example of a control signal S for controlling the electrically operated valve as a function of time, and the curve of gas pressure P delivered at the outlet of the valve receiving this control signal. FIG. 1a depicts the control signal S which has a first high level during periods T1, known as the open level, and a low level during periods T2, known as the closed level. The periods T1 and T2 are usually equal. FIG. 1b depicts the pressure of the gas at the outlet of the valve in a temporal relationship with the control signal S. The pressure level corresponding to the closed control signal has been labelled C and the pressure difference between the open and closed signals has been labelled Q. It can be seen from this figure that during the periods corresponding to the application of the open signal, the pressure is not strictly in the shape of a square wave but has an inclined rising edge F1, a falling edge F2 which is also inclined and, while the open signal is applied, the pressure is not constant. As has been mentioned, it is desirable for the shape of the pressure waves to be as rectangular as possible.
Another problem in supplying a pulsed flow lies in the fact that these valves are used and controlled a great many times during the period that the burner is operating. It is therefore necessary that the valve should not only be as near as possible to a perfect square wave, but also for it to have very good repeatability in terms of the opening pressure and closure pressure of the fluid delivered over time.
In an attempt at solving this problem, a valve described in particular in American Patent U.S. Pat. No. 5,222,713 has already been proposed. The flow control element of this valve consists of a part whose periphery is deformable, thus making it possible, depending on the stress applied to it, to allow the fluid to pass or to interrupt its passage. The actuator allowing the pulsed deformation of this component is, for example, a piezoresistive element controlled electrically according to the desired pulsation frequency. However, it has become apparent that the deformation of the element constituting the shutter element of the valve alters with use and is not very repeatable from one valve to another, particularly as far as the flow rates corresponding respectively to the open and to the closed states are concerned.
One object of the present invention is to provide a controllable valve, particularly for delivering a pulsed flow, which has an outlet curve in terms of flow or in terms of pressure which is approximately in the form of rectangular square waves and which, moreover, has satisfactory repeatability, particularly as far as the flow rate or pressure supplied in the open state and in the closed state are concerned.
In order to achieve this objective according to the invention, the controllable valve particularly for delivering a pulsed flow of fluid, comprises:
a valve body;
a valve seat dividing the inside of the valve body into a fluid inlet chamber and an outlet chamber;
a valve shutter element capable of moving in one direction of travel to collaborate with the valve seat;
an actuator comprising a stationary control part for receiving control signals and a moving part, the said stationary part applying to the moving part a force which corresponds to the control signal;
first rigid means of connection extending in the direction of travel so as to connect the said moving part of the actuator to the said valve shutter element;
a mechanical stop;
a member that can be compressed under the effect of a force applied to it, comprising a first end secured to the said mechanical stop; and
second rigid means for dynamically connecting one of the faces of the said valve shutter element to the second end of the said compressible member.
It will be understood that, on the one hand, since the open and closed flow rates respectively are defined by a rigid seat and by a rigid valve shutter element, these flow rates are intrinsically perfectly stable over time. It will also be understood that, when the control signal is no longer applied to the stationary part of the actuator, the shutter element moves in one direction or the other depending on the embodiment in question, not only under the effect of the cancellation of the corresponding force but also under the effect of the release of the compressible member which was previously compressed. It will be understood that by using a compressible member which has properties which are very stable over time, it will be possible to obtain very uniform valve operation. Furthermore, it is understood that the rising or falling edges will be improved by comparison with the known solutions, because of the action of the compressible member
According to a first embodiment, the second rigid means of connection connect to the second end of the compressible member that face of the valve shutter element which faces towards the valve seat.
According to a second embodiment, the second rigid means of connection connect to the second end of the compressible member that face of the valve shutter element which does not face towards the valve seat, the said second rigid means including the said first rigid means of connection.
It will be understood that, according to the first embodiment, in the absence of a control signal, the valve shutter element returns spontaneously to its open position under the effect of the compressible member. By contrast, in the second embodiment, the valve shutter element returns to its closed position under the effect of the release of the compressible member. As will be indicated later on, the term xe2x80x9cclosed positionxe2x80x9d must not be taken as necessarily meaning that the shutter element is pressed against its seat in such a way that the flow rate is effectively zero, but as meaning a position of the shutter element such that the flow rate supplied is low by comparison with the flow rate supplied in the open position.
As a preference, the compressible member consists of a part made of elastomeric material chosen for the consistency of its compressibility characteristics, this part having two parallel faces which are interposed directly or indirectly between the mechanical stop and the shutter element.
The invention also relates to a method of combustion in which a flow of oxidizing agent and a flow of fuel are injected into a furnace, in which the oxidizing agent and the fuel react with one another to produce a flame capable of heating a charge. According to the invention, this method is characterized in that the flow of oxidizing agent and/or the flow of fuel is or are injected in a pulsed manner using a pulsing valve as described in the text of this specification.
As a preference, at least one pulsing valve is used to inject fuel and at least one pulsing valve is used to inject oxidizing agent, the pulsations being identical (or different) in terms of duration but in phase opposition. According to another alternative form of the invention, the pulsations have the same duration (or different durations) but are in phase.
According to another alternative form of the invention, in which there are at least two separate injections of oxidizing agent, using identical or different oxidizing agents chosen from oxygen, substantially pure oxygen, (and particularly oxygen delivered by an apparatus for separating the gases in the air, operating by adsorption, also known as VSA or xe2x80x9cvacuum swing adsorptionxe2x80x9d, particularly containing at least 88% of oxygen, about 2 to 5% of argon, and any remainder being 0 to 10% of nitrogen) oxygen-enriched air, air or oxygen-impoverished air, at least one of the two injections being carried out using a pulsing valve. In general, the invention also relates to the use of a pulsing valve as defined in this specification for pulsing an oxidizing gas and/or fuel.